No, it's fine. The lens will project an image that is larger than the sensor, so you won't see the same field of view for that particular lens as you would on a full frame camera, but it will otherwise work normally.

In theory, you might expect a little more lens flare due to the extra light bouncing around inside the camera, but that's probably of theoretical concern only.

Full frame is the defacto standard for focal length. on full frame 50mm is considered a "normal lens' meaning it is not wide angle or telephoto and provides a depth of field similar to the human eye in terms of the percived distance between objects in frame (for further understanding wideangle lenses expand depth of field making objects appear farther from eachother and telephoto lenses compress objects).

on a t3i which has an APS-C sensor there will be a 1.5x "crop factor" meaning the 50mm lens will provide a depth of field and angle of view similar to a 75mm lens on a "full frame" sensor.

because the lens has a wider image circle ment for a full frame sensor it will always "cover" the smaller aps-c sensor. But if you take a lens designed for an APS-C sized sensor and place it on a full frame camera you are likley to see heavy vingetting or literally see a black ring around the image because the lens won't project an image big enough to cover the entierty of the sensor.

Actually, in movie terms, Academy 35mm (or more recently, Super 35mm) is the standard. Full Frame has only become relevant to film production (other than VistaVision cameras), with the advent of dSLRs and the ever increasing sensor size of RED cameras

In the stills world, 35mm may be the most widely used format, but it is hardly the 'standard'.

I don't quite understand the whole full frame aspect of cameras, or sensors for that matter. Is there much difference when I use a full frame lens on a T3i?

To answer your question... yes the 'coverage' of a lens designed for a 'full frame' 35mm Still negative, will cover a smaller sensor such as the T3i.

To answer a question that probably has not really occurred to you... get an iPhone or Android app that calculates for a given camera and sensor, what the important parameters are for that lens with that sensor.

The most important, for 'framing' is Angle of View or Field of View. This will tell you if you need to go down or up in focal length for that sensor/camera. The other feature of these tools is they usually also have 'Depth of Field' values, so you can know that say if your subject is 6 feet from the camera, at F/4 that the subject will be in focast from nose to the ears, etc.

Actually, in movie terms, Academy 35mm (or more recently, Super 35mm) is the standard. Full Frame has only become relevant to film production (other than VistaVision cameras), with the advent of dSLRs and the ever increasing sensor size of RED cameras

In the stills world, 35mm may be the most widely used format, but it is hardly the 'standard'.

This is sort of why I have been objecting to the whole 'crop' thing, relative to 35mm Still film negative size, and relating it to Motion Pictures.

I know that when I first got into still photography, there was no 'crop factor' when I moved between 35mm still, 2 1/4 x 2 1/4, or in my case, because I'm weird... 2 1/4 x 3 1/4, and then when I got a 4x5 view camera, again no mention of relating the lens to 35mm still negatives...

It is of course a conjecture on my part what Motion Picture photographers may have done when moving around the various 35mm Motion Picture formats... My guess is most looked up a table or the like, or just because they had shot with someone else as DoP, 'acquired' an understanding of the lens and format.

There was one rule, that is the 'normal' lens was the length of the negative diagonal... but even there, there wer deviations... for example, using that formula 150mm (approx) was the diagonal for 4x5... but I preferred the 'look' of a 210mm lens...

The issue of coverage is a bit different, because of the often 'unique' mounts for lenses, people tended to by the particular manufacturer's lenses, or some 3rd party supplier which had the same mount, and attendant 'coverage'.

One could in to trouble with 4x5, because one could move the film plane around with swings, tilts, etc.

I think something to keep in mind is that 'Full Frame' refers to still photography where the 35mm film runs horizontally 8 sprocket holes 'wide' which is basically VistaVision in the motion side of things. Traditional movie cameras run the film vertically 4 sprocket holes 'tall.' It's important to know that in film this made a huge difference in resolution because you used almost twice as many film grains to capture the image. Digitally however you are captureing the same amount of pixels... APS-C and Full Frame are both going to be 1080p or 4K or whatever. But because of the physical size difference in the sensor the field of view will be different... a 50mm on a Full Frame 1080p camera will be 'wider' that the same lens on a APS-C 1080p camera. There are some other differences , particularly in how the cameras sample the 'extra pixels' to create whats basically a 2 megapixel image.

So to summerize, I would say yes there is a significant difference. But it's a difference that doesn't really matter as long as you are aware of how it affects the image.

example... I just finished shooting a very low budget webseries on a blackmagic pocket. I used my pentax K mount lenses and the widest one I had was a 30mm... which is a 'normal' on something like the BMPCC... so I got a thing called a focal length reducer which is an adapter that allowed that lens to function as a 'wide' on the black magic pocket. I now had lenses that covered wide, normal, and long... so problem solved a far as I was concerned.

I don't quite understand the whole full frame aspect of cameras, or sensors for that matter. Is there much difference when I use a full frame lens on a T3i?

What specifically don't you understand?

Sensors come in different sizes, and lenses made for the larger ones have to project a larger image out of the back of the lens to fill the sensor. In terms of focal length, that's just a physical measurement of the lens and doesn't change no matter what sensor the lens is mounted in front of, but a 50mm lens (for example) made for a smaller sensor camera will not project as large an image circle as a 50mm lens made for a larger sensor camera and thus vignette on that large sensor.

So generally a lens made for a larger sensor will work on a smaller sensor but may or may not vignette if you go the other way, put a lens made for a smaller sensor onto a camera with a larger sensor.

"Full-Frame" is a still photography term and refers to the traditional 35mm still camera frame that is 36mm x 24mm, on 8-perf 35mm running sideways. The equivalent in motion picture work is VistaVision, which is also 8-perf 35mm horizontal. The traditional 35mm movie frame was originally 24mm x 18mm, 4-perf 35mm vertical, what in still photography was called a "Half-Frame".

The APS-C sensor size in digital still cameras is the closest thing to the width of a 35mm movie frame. Height-wise, most are similar to 3-perf 35mm cine.

The sensor size affects how much of the lens image is cropped, so the same focal length lens will have half the horizontal view if switched to a sensor that is half the width, what would be called a 2X crop. Most people use the diagonal dimension of the sensor to determine crop factor but this is confusing because different aspect ratios will produce different diagonal measurements, so I generally compare either the horizontal or the vertical dimensions.

So if comparing a sensor that is 36mm wide (Full-Frame 35) to one that is 24mm wide (Super-35 cine / APS-C more or less), you'd divide 36 by 24 and get a 1.5X crop factor. This is useful if you are trying to calculate equivalency of field of view, i.e. what focal length to use on a 35mm movie camera, for example, to match the view of a 50mm lens on a FF35 / VistaVision camera (divide 50 by 1.5 and get 33.33333....)

However, if you are used to shooting in 35mm cine, then you already know what the view of different focal lengths are for that size of film or sensor. This is one advantage to APS-C cameras, the focal lengths used are more or less the same as with 35mm cine -- an 18mm lens, for example, gives you the same wide-angle view on either an APS-C camera or a 35mm movie camera.